The invention relates to the field of optical fiber transmission, and more specifically, the chromatic dispersion compensation and the chromatic dispersion slope compensation in transmission systems by optical fiber.
For the optical fibers, generally the index profile is qualified as a function of the shape of the graph of the function which links a radius of the fiber to the refractive index. In a standard manner abscises represent the distance r to the centre of the fiber and ordinates represent the difference between the refractive index and the refractive index of the fiber cladding. It is thus referred to the index profile as “step”, “trapezoid” or “triangle” for graphs which have step, trapezoid or triangle respective shapes. These curves are generally representative of the theoretical shape or reference profile of the fiber, the fiber manufacturing constraints can lead to a substantially different profile.
An optical fiber is typically composed of an optical core, having the function of transmitting and possibly amplifying an optical signal, and an optical cladding, having the function of confining the optical signal in the core. For this reason, the refractive indexes of the core nc and of the outer cladding ng are such that nc>ng. As commonly known, the propagation of an optical signal in a single-mode optical fiber is divided in a guided fundamental mode in the core and into guided secondary modes over a certain distance throughout the core-cladding assembly, called cladding modes.
In new high bit-rate wavelength-division-multiplexing transmission networks, it is advantageous to manage the chromatic dispersion, notably for bit rates greater than or equal to 40 Gbit/s. The purpose is to obtain, for all the wavelength values of the multiplex, a substantially nil cumulated chromatic dispersion over the link so as to limit the broadening of pulses. “Cumulated chromatic dispersion” is called the integral of the chromatic dispersion over the length of the fiber; with constant chromatic dispersion, the cumulated chromatic dispersion is equal to the product of the chromatic dispersion and the fiber length. A cumulated value of a few tens of ps/nm for the dispersion is usually acceptable. It is also beneficial to avoid, in the vicinity of wavelengths used in the system, nil values of the local chromatic dispersion, for which the non-linear effects are more important. Finally, it is also advantageous to limit the cumulated chromatic dispersion slope over the multiplex range so as to avoid or limit the distortions between the multiplex channels. This slope is usually the derivative of the chromatic dispersion in relation to the wavelength.
Typically single-mode fibers (SMF) with step index profiles or dispersion shifted fibers, also known as non-zero dispersion shifted fiber (NZDSF+) as line fiber for fiber-optic transmission systems. NZDSF+ is qualified as dispersion shifted fibers having a non-zero and positive chromatic dispersion for the wavelengths for which they are used, typically about 1550 nm. These fibers have, for these wavelengths, a low chromatic dispersion typically less than 10 ps/(nm.km) at 1550 nm and a chromatic dispersion slope between 0.04 and 0.1 ps/(nm2.km).
To compensate for the chromatic dispersion and the chromatic dispersion slope in SMF and NZDSF+ fibers used as line fibers, one can use small lengths of dispersion compensating fiber (DCF). In the choice of DCF fiber, one tries to get the ratio of the chromatic dispersion over the dispersion slope of the compensating fiber to be substantially equal to that of the line fiber. This ratio is designated dispersion over slope ratio (DOS). The smaller the DOS ratio is of a transmission fiber the harder it is to compensate the dispersion and the dispersion slope with a single compensating fiber DCF.Yet, some NZDSF fibers, for example the fiber sold by the Corning company under the e-leaf® brand, have a very small DOS, of about 50 nm. As these line fibers are used in high bit rate long distance transmissions, it is important to be able to compensate the cumulated chromatic dispersion of these fibers with a compensation fiber having a similarly small DOS value. Yet, such a DOS value, as small as 50 nm, is difficult to attain in a DCF fiber.
For example, U.S. Pat. No. 6,587,627 proposes a compensating fiber profile with a central core, a buried inner cladding, a ring and an outer cladding, having a DOS value at 1600 nm of about 60 nm that can attain 50 nm. This type of profile either has the inconvenience of too great a chromatic dispersion at 1550 nm, greater than −50 ps/nm/km, or too great a DOS value at 1550 nm, greater than 70 nm.
WO-A-03/050577 proposes a compensating fiber profile with a central core, a buried inner cladding, a ring and an outer cladding, having a DOS value at 1550 nm of about 50 nm. This profile type does not allow to attain acceptable bending losses for putting into module.
WO-A-03/050578 proposes a compensating fiber profile with a central core, a buried inner cladding, a landing, a ring and an outer cladding, having a DOS value of between 60 and 115 nm for wavelengths of between 1550 and 1610 nm. This profile type has the inconvenience of high cut-off wavelengths; far greater than 1650 nm.
US-2004/0234219 proposes a dispersion compensating optical fiber with a central core, a first burried inner cladding, a ring and a second shallowly burried inner cladding and a outer cladding.
JP-2003/270471 proposes a dispersion compensating optical fiber with a central core, a first burried inner cladding, a ring and a second shallowly burried inner cladding and a outer cladding.
WO-A-01/71391 proposes a compensating fiber profile with a central core, a buried inner cladding, a ring and an outer cladding, having a DOS value at 1550 nm of between 40 and 100 nm. This profile type has the inconvenience of high cut-off wavelengths; far greater than 1650 nm.
Furthermore, EP-A-1 308 756 discloses a compensating fiber profile with a central core, a first buried inner cladding, a ring, a second buried inner cladding and an outer cladding, having certain DOS values close to 50 nm at 1550 nm. This profile type has the inconvenience of too high a chromatic dispersion, greater than −50 ps/nm/km, and does not allow to attain the desired compromise, notably between effective surface and bending losses.
EP-A-1 170 604 discloses a compensating fiber profile with a central core, a first buried inner cladding, a ring, a second buried inner cladding and an outer cladding, having a DOS value at 1550 nm between 30 and 66 nm. The proposed fibers do not allow to attain the desired compromise because the second buried inner cladding is insufficiently buried; notably this fiber profile does not allow to obtain acceptable bending losses for the desired characteristics.
It is noticed that the majority of the disclosed profiles of the fibers do not enable to easily attain a DOS value as small as 50 nm.
Moreover, it is noticed that the DCF compensating fibers, having a small DOS, are not optimised in order to be used in dispersion compensating modules. These dispersion compensating modules are generally placed in optical repeaters along the transmission line; the DCF compensating fiber is coiled in the module and is to compensate the cumulated dispersion and the cumulated dispersion slope over several hundred kilometres in the line fiber. Yet, the DCF compensating fibers having a small DOS often have high bending losses and/or a high cut-off wavelength and/or a small effective surface and/or a small figure of merit (FOM).
None of the profiles of the fibers of the prior art cited and analysed above attain an optimal compromise between a limited DOS value and acceptable figure of merit, bending losses, cut-off wavelength and effective surface characteristics.
The majority of currently commercialised modules for compensating chromatic dispersion over the C band of a line fiber having a small DOS, of about 50 nm, comprise a DCF compensating fiber with a higher DOS, of about 60 or 70 nm coupled to an additional fiber of SMF type in order to increase the global DOS of the line fiber to compensate it. Such a solution is unsatisfactory as it consists in degrading the losses, the polarisation mode dispersion (PMD) and the behavior of the non-linear effects of the module in order to improve the chromatic dispersion compensation and the dispersion slope compensation of the line fiber by the same module. Furthermore, this solution complicates the manufacturing of modules, increases their manufacturing cost and renders them non-symmetric.
There is therefore a need for a chromatic dispersion compensating fiber which has a small DOS value and that can be directly used in a compensating module without any additional fibers; that meaning a dispersion compensating fiber which has an optimal compromise between a small DOS and a high figure of merit, small bending losses, a small cut-off wavelength and an enlarged effective surface.